1. Technical Field of the Invention
This invention relates to methods and techniques for reducing the visual impact of cell gap and drive voltage nonuniformities in liquid crystal displays, and more particularly to projection and other magnified displays based on liquid crystal on silicon microdisplays.
2. Discussion of Related Art
Liquid crystal displays and more particularly liquid crystal on silicon microdisplays are very sensitive to variations in cell gap thickness, pretilt and drive voltage. The effects of these variations can be observed as differences of intensity seen in regions where such differences are noticeable. These same phenomena exist in all liquid crystal displays but often the distance over which the nonuniformities are manifested are quite small compared to the overall display. Additionally there are methods available to solve this problem that are not suitable in the microdisplay environment.
The present problem is the one of nonuniformities in microdisplays used in displays that magnify the images created by the microdisplays. Nonuniformities within the display are magnified in the same way that the images themselves are magnified. The nonuniformities typically manifest themselves over a range of 50 to several hundred pixel elements and thus are visible but relatively slow changing phenomena.
In flat panel displays the problem of variations in cell gap is shown in FIG. 1. The cell gap problem may be addressed by using spacer balls or spacer rods in the active area of the display (see FIGS. 2a and 2b). These spacers place a minimum bound on the spacing between the two substrates that keeps the distance relatively uniform over the very large area, often on the order of 11 inches diagonal or more, of the display.
Spacers are undesirable in certain display applications and have proved problematic in liquid crystal on silicon display. The use of random spacer balls has been evaluated at great length and found to be unacceptable. Randomly placed spacer balls block the primary color at that point on the microdisplay, invariably create small spots in the projected image where the remaining two of the three primary colors are displayed. The spots show as areas where complementary colors are visible within fields of otherwise white light. While this problem exists to a small degree in direct view panels, the effects are normally negligible, whereas the effects in the magnified images of projection displays become objectionable and threaten the commercial success of the product.
Several solutions exist. It is possible to align all the spacer posts by building them into the backplane. This is not a complete solution because the three microdisplays are normally aligned using a combination of mechanical alignment and electronic image convergence. Alternatively the microdisplays can be constructed without the use of spacers of any type. While preferable, this leads back to the fundamental problem of uniformity across the aperture of the display device. An analysis of the visible effects of these nonuniformities is in order.
These nonuniformities normally arise as part of the manufacturing processes used for these displays. For example, in liquid crystal on silicon microdisplays the surface of the microdisplay is rendered local flat and optically reflective by a process called chemical-mechanical polishing, or CMP. It is well know that CMP sometime results in a differential ablating of the original surface material. While the resulting surface is much better than the original surface it still is not as flat as a piece of highly polished glass. Local variations result in a surface which, when integrated into a display, results in perhaps a 5% variance in the thickness of the liquid crystal layer that is being driven so as to modulate light.
Other sources of variance include a nonuniform rubbing to create alignment of the liquid crystal. In such cases a slight change in rubbing density due to surface topology can create a slight difference to the liquid crystal pretilt which in turn can change the effective birefringence of that part of the cell and thus result in a nonuniformity in the cell.
An additional source of variance is the delivery of nonuniform voltages to the pixel electrodes associated with a image. This can result from a variety of factors. Common causes include improper or nonuniform line impedance matching, use of low cost CMOS digital to analog converters without calibration, and lack of uniform and consistent pixel capacitor size in DRAM based microdisplays manufactured in CMOS processes.
In the case of an SRAM based display the liquid crystal display is modulated by pulse width modulation because the logic cell selects a high state or a low state. In practice in the example of a normally black mode twisted nematic liquid crystal device, there are two “low” states that are close to the voltage of the common electrode and two “high” states that are further away from the voltage of the common electrode. It is desirable when driving nematic liquid crystals that these be mirror images of each other and that the alternation take place at a relatively high rate. If two pixel electrodes are driven by the same set of pulse width modulated data then the RMS voltage associated with the two pixel electrodes will be identical. If the cell gaps associated with the two pixel electrodes differ from each other by some margin, say 5%, then there will be a corresponding difference in the field strength across the pixel gap as a function of distance. As a result, the pixel electrode associated with the greater of the two cell gaps will need to see a higher RMS voltage in order to achieve the same level of birefringence in the associated liquid crystal as is seen in the liquid crystal associated with the pixel electrode associated with the lesser cell gap. This greater RMS voltage can be achieved only by driving the pixels electrode for a greater period of time with the “high” state voltages.
The impact of all these variations on the optical throughput of a given microdisplay can be quite pronounced. For example, in liquid crystal on silicon displays using the twisted nematic electro-optic effect an increase in the thickness of the cell results in a smaller change in the optical state of the liquid crystal relative to adjacent regions in the same device where the cell gap is slightly lower. An analysis of the voltage transfer curves of the two regions, where optical throughput is plotted as a function of the drive voltage across the cell, reveals similar but not identical curves. In both cases the effective gray scale region in the thicker cell demonstrates a need for high voltages to achieve full optical efficiency when compared with the curve for the thinner cell.
Measuring the effects of these nonuniformities across the pixel array of the microdisplay requires an instrumentation device that can collect segments of the voltage transfer curve as a function of position on the display. Any number of devices can be devised to collect this data. One commercially available automated device that is particularly well suited to this task is the MicroDisplay Inspection System (MDIS) recently developed by Westar Corporation of St. Louis, Mo. This capability is described in a set of brochures downloaded from their website http://www.displaytest.com/mdis/detailed.html on Apr. 30, 2002.